As regards the heat treatment of liquid or pumpable foods, use can be made of steam in order to heat the food product in rapid and efficient manner. One of the methods employed for injecting steam direct into the product is to use an injector.
Depending upon the temperature to which it is desired to heat the product, it is possible to obtain a pasteurised or aseptic product, or alternatively a product possessing extended shelf life in cold storage. The commonest method is to heat the product which, for example, may be dairy produce, juices, viscous fruit products or the like, to a temperature where a total destruction of harmful micro-organisms is obtained, this gives a food product which keeps stable on storage at room temperature. No unbroken refrigerated chain from storage is necessary, which may be an advantage, particularly in the developing countries.
Direct heating of the product by injecting steam into the product gives a rapid and efficient heating. As a result of this rapid method, it is possible to reduce the treatment time, which in total gives a reduced heat action on the product and a product will be obtained which maintains a higher level of quality, above all in terms of flavour.
There is a large number of injectors on the market which all display similar construction, with an inlet for the product which is to be treated and an outlet for the ready-treated product. The injector further has an inlet for steam under high pressure which, through various arrangements with gaps or ducts, is caused to intermingle with the product and which heats the product to the desired temperature. Depending upon the construction of the injector, with gaps, or ducts, it may be suitable for products possessing different viscosities.
European Patent Specification EP 839 461 describes an injector particularly suited for viscous products. Within the injector housing, there is a thick-walled pipe with a large number of through-going channels or ducts. The product passes inside the pipe and steam is supplied through the ducts. Outside the thick-walled pipe, there is a control sleeve which may displaced in the longitudinal direction of the pipe in order, by such means, to regulate the quantity of supplied steam and thereby regulate the temperature to which the product is to be heated. The thick-walled pipe is manufactured from some plastics material or a ceramic, while the control sleeve is manufactured from stainless steel.
The disadvantages inherent in an injector possessing a construction as described above is that there is an inner pipe with a high coefficient of thermal expansion and a surrounding sleeve which has a considerably lower coefficient of thermal expansion. For such an injector to be easy to regulate, there must be a relatively large clearance between the parts, with the result that steam leaks out between the parts and a much poorer and less distinct regulation of the injector is obtained. In that the product passes through the pipe with a number of small ducts, there is also the risk that the product “migrates out” in the ducts and the injector thereby becomes more difficult to clean, which is a major disadvantage for equipment for the food industry where extremely high demands are placed on hygiene.